Carbon with its electron configuration of [He]2s22p2 and unrivaled ability to form different hybridization states, i.e., sp, sp2, sp3 or their mixtures, can exist in various allotrope forms (e.g., diamond, graphite, graphene, carbon nanotubes, etc.), with many of the forms possessing unique sets of properties. Diamond and graphite are some of the best known examples. Yet, while diamond consists of sp′-hybridized carbon atoms and strong covalent bonds, graphite exhibits a highly anisotropic layer structure, with honey comb-like atomic planes of strongly-bonded sp2 hybridized carbon atoms and very weak van der Waals interactions between the planes. Indeed, a single layer of graphite known as graphene is yet another allotrope of carbon that possesses various superior characteristics.
Among well-known carbon allotropes are also fullerenes, the simplest one being C60, a nanosphere approximately 1 nm in diameter and consisting of 60 carbon atoms, each covalently bonded to its three neighbors. Carbon nanotubes (buckytubes) are elongated cylindrical fullerenes characterized by high aspect ratio and nanometer-sized diameters. As such, carbon nanoshells are attractive for applications in the areas of catalysis, energy technologies, and bio-medical fields. However, fullerenes are often too small for many desired applications, containment of drug molecules being one of them. There have been attempts to make larger carbon nanoshells, but the attempts have been met with limited success due to the relatively poor controllability of the number of layers, size, and the coalescence of the shells.
Related to fullerenes are also carbon nanocages (CNCs) which are a type of spherical nanocarbon with graphitic shells. While typical fullerenes exist primarily as isolated nanostructures, carbon nanocages may form a three-dimensional (3D) network, where individual hollow single- or few-layer nanoshells are interconnected to form large-scale structures, sometimes even in the millimeter or centimeter scale. Thus, in recent years, carbon nanocages have attracted significant attention due to their unique properties and promising applications.
With respect to the possible applications, because of their highly porous structure, carbon nanocages are considered for gas separation as membrane materials and also, due to their considerable pore volume, for gas storage and biomedical applications, including drug delivery systems. Furthermore, because of high open surface area, carbon nanocages are also, similarly to other 3D meso- and nanoporous carbons, promising materials for sensing, catalysts and catalysts supports, as well as for electrodes for energy storage devices. In fact, one of the promising applications of carbon nanocages is to use carbon nanocages as electrode materials in supercapacitors, electrochemical storage devices that exhibit high power capabilities and play a key role in the development of several important technologies, including electric transportation, energy management systems, and intelligent wireless sensors systems.
Various carbons, such as carbon nanotubes, activated carbon, mesoporous carbon, and graphene, have been demonstrated as promising supercapacitor electrode materials. Owing to their high surface area, however, carbon nanocages are particularly suitable for the use in electrochemical double layer supercapacitors (EDLS), which utilize the energy storage mechanism based on the physical adsorption of charges at the electrode-electrolyte interface. Carbon nanocages can also be used in so-called pseudocapacitors because, similar to other carbonaceous materials, carbon nanocages can be doped with nitrogen, which results in additional charge storage capability, known as pseudocapacity. Application of carbon nanocages in so-called hybrid supercapacitors, where one electrode is based on electrochemical double layer capacity and the other one on pseudocapacity, can lead to a further increase, even doubling in optimal cases, of the effective capacity.
Recently, nitrogen-doped carbon nanostructures (e.g., carbon nanotube, carbon nanofibers) were further examined for different electrochemical reactions including CO2 reduction, oxygen reduction, and hydrogen evolution reactions (HER). HER, a cathodic half reaction of water splitting, is an electrochemical reaction where protons are reduced to form hydrogen. Thermodynamically, the reaction takes place at 0.0 V versus a NHE (normal hydrogen electrode) at pH=0. However, additional energy is needed to surmount a certain activation energy barrier (known as overpotential) to occur. Therefore, electrocatalysts are essential to lower the overpotential, and consequently promote the reaction rate and efficiency. The kinetics and onset potential for HER is governed by the intrinsic properties of the electrocatalysts.
For commercial application, the HER catalysts must hold the following key characteristics: high catalytic activity; long term durability; low price; and stability in different electrolytes. Theoretical and experimental studies have confirmed that among HER catalysts, noble metals (e.g., Pt, Pd etc.) are some of the most active and Pt is the most efficient and capable to drive HER at the lowest potential. For instance, 10 mA/cm2 current can be attained for a Pt electrode at as low as 0.1 V vs RHE working cathodic potential in acidic condition (pH 1). However, due to its high price and poor stability in the presence of contamination (e.g., CO2), Pt is not an ideal candidate for commercial applications using electrolysis hydrogen generation for renewable energy storage. Metal-free catalysts are inexpensive and possess significantly high catalytic activity and strongly compete with noble metal catalysts and even surpass their activity in case of CO2 electrochemical reduction. However, the development of N-doped carbon nanostructures (e.g., carbon nanotube and graphene) has been a complex process due to an additional nitrogen atom insertion step, requirement of high temperature and pressure, and poor control over doping level and physical properties (e.g., size, conductivity, etc.) of nanostructures.
To date, several synthesis methods have been proposed to obtain carbon nanocages. Some investigators have used spray pyrolysis of iron carbonyl and a carbon precursor, including N-containing carbon precursors for in situ fabrication of N-doped carbon nanocages, and have produced nanocages and used the nanocages as support for a Pt catalyst with remarkable catalytic activity and stability towards hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). Recently, others have demonstrated that N-doped carbon nanocages alone can also serve as efficient and stable metal-free ORR electrocatalysts. Such catalysts showed excellent performance compared to commercial Pt/C electrocalatysts and also exhibited high stability towards methanol crossover and CO poisoning. The N-doped nanocages used, have been obtained by in situ MgO template method with pyridine as the precursors. In previous studies, investigators have demonstrated analogous methods based on a benzene precursor. Still others have prepared carbon nanocages using an in situ MgO template method and have demonstrated excellent properties for supercapacitor electrode materials. Recently, certain investigators have used direct carbonization of non-permanent highly porous MOFs to obtain nanoporous carbons with high surface area and good physicochemical stability. Further investigators have synthesized carbon nanocages using nickel oxalate and citric acid in a stainless-steel autoclave at 550° C. Although the nanocages were of 200-500 nm diameter, electrochemical performance was reported to improve only after annealing at 600° C. for 5 h. Additional groups have used laser-induction complex heating evaporation to produce carbon-coated iron nanoparticles (5-50 nm) with a few layers of graphitic shells, and have indicated the same process can be extended to other metals such as Ni and Co for synthesis of nanocages.
To date, however, and for many applications, there still remains a need for methods that allow carbon shells to be fabricated with determined size, number of layers, and desired dispersion. A simple, scalable process that can utilize less expensive materials in a shorter time frame would also aid in commercialization efforts for nanocage applications.